Nowadays, internal combustion engines, for example, those using a heat cycle such as an Otto cycle or a Diesel cycle, are widely used as common mechanical power sources. The problem here is that such internal combustion engines, as by polluting the atmosphere with the exhaust gas they emit, and by producing noise, cause various public hazards, which have been provoking great social controversies.
On the other hand, refrigerators and the like generally adopt a vapor compression refrigeration cycle which uses a chlorofluorocarbon refrigerant as a working gas to obtain intended cooling performance through evaporation and condensation thereof. The problem here is that chlorofluorocarbons are chemically highly stable, meaning that, once discharged into the atmosphere, they reach the stratosphere and destroy the ozone layer. For this reason, the use and production of specified chlorofluorocarbons are now restricted.
It is under this background that, free from the problems mentioned above, Stirling engines using a Stirling cycle or a reverse Stirling cycle have been receiving increasing attention.
A Stirling engine using a Stirling cycle is an external combustion engine, and thus offers the following advantages: it does not require any specific type of heat source; and it is less likely to produce hazardous substances because, even if a fuel is combusted, it is not combusted under high-temperature, high-pressure conditions.
A Stirling engine as described above uses an environment-friendly gas, such as helium gas, hydrogen gas, or nitrogen gas, as a working gas.
On the other hand, a Stirling refrigerating unit using a reverse Stirling refrigeration cycle is known as a type of compact cryogenic refrigerating unit.
FIG. 7 is a side sectional view of a free-piston Stirling refrigerating unit as one example of a Stirling engine.
The Stirling refrigerating unit B comprises a pressure container 1, a cylinder 2 secured inside the pressure container 1, a power piston 3 and a displacer 4 provided inside the cylinder 2. The power piston 3 and the displacer 4 are arranged on the same axis, and reciprocate linearly along the axis.
The displacer 4 comprises a displacer piston 41 and a rod 42. The rod 42 is placed through a slide hole 31 formed at the center of the power piston 3, and the power piston 3 and the displacer piston 41 can slide smoothly along an inner circumferential surface 21 of the cylinder. The power piston 3 is elastically supported on the pressure container 1 with power piston supporting springs 5, and the displacer 4 is elastically supported on the pressure container 1 with a displacer supporting spring 6 via the rod 42.
The space inside the pressure container 1 is divided by the power piston 3 into two spaces. Of these two spaces, one is a work space 7 located on the displacer 4 side of the power piston 3, and the other is a back-pressure space 8 located on the side of the power piston 3 opposite to the displacer 4. These spaces are filled with a working gas such as high-pressure helium gas.
The power piston 3 is made to reciprocate with a predetermined cycle time by a piston drive body (here, a linear motor 9). This causes the working gas to be compressed and expanded in the work space 7. The displacer 4 is made to reciprocate linearly by the difference in pressure between in the work space 7 and in the back-pressure space 8. Here, the power piston 3 and the displacer 4 are set so as to reciprocate with the same cycle time but with a predetermined phase difference. As the result of the power piston 3 and the displacer 4 reciprocating with a predetermined phase difference, a reverse Stirling refrigeration cycle is achieved. So long as the operating conditions remain the same, the phase difference is determined by the mass of the displacer 4, the spring constant of the displacer supporting spring 6, and the operating frequency of the power piston 3.
The work space 7 is further divided by the displacer piston 41 into two spaces. Of these two spaces, one is a compression space 71 surrounded by the power piston 3, the displacer piston 41, and the cylinder 2, and the other is an expansion space 72 surrounded by one end of the cylinder 2 and the displacer piston 41. The compression space 71 is where heat is produced, whereas the expansion space 72 is where cold is obtained.
The principle of a reverse Stirling refrigeration cycle, including how it produces cold, is widely well-known, and therefore, in such regards, no description will be given in the present specification.
The displacer 4 uses the pressure difference between in the compression space 71 and in the back-pressure space 8 as a drive source for the linear reciprocating motion, and achieves the reciprocating motion by exploiting the resonance between the displacer 4 and the supporting spring 6. A flow of the working gas through the slide hole 31 between the work space 7 and the back-pressure space 8 causes a flow loss, which reduces the efficiency of the Stirling engine. Thus, to prevent the engine efficiency from being reduced due to the working gas flowing through the slide hole 31, it is preferable that as small a diametral clearance as possible be left between the inner circumferential surface of the slide hole 31 and the outer circumferential surface of the rod 42.
The output (freezing performance) of a free-piston Stirling engine can be increased by increasing the resonance frequency of the displacer 4.
The aforementioned operating frequency increases as the aforementioned resonance frequency increases, and this can be practically achieved by increasing the resonance frequency of the displacer. The resonance frequency is determined by the mass of the displacer 4 and the spring constant of the spring 6 elastically supporting the displacer 4. Thus, to increase the resonance frequency of the displacer, it is necessary, for example, to reduce the mass of the displacer 4 or to increase the spring constant of the spring 6.
The displacer 4 uses the pressure difference between in the compression space 71 and in the back-pressure space 8 as a drive source for the linear reciprocating motion, and a force in the axial direction acts on the rod 42 facing the back-pressure space 8. Reducing the outer diameter of the rod 42 in an attempt to make the displacer 4 lighter results in reducing the stiffness of the rod 42. This makes the rod 42 likely, while reciprocating repeatedly, to be deformed by a force acting thereon in the axial direction. Even a slight deformation in the rod 42 may cause the rod 42 to come into contact with the slide hole 31 because of the small clearance between the rod 42 and the slide hole 31, producing sliding friction where they come into contact with each other. The sliding friction hinders stable reciprocating motion of the displacer 4 and the power piston 3, thereby reducing the efficiency and reliability of the Stirling engine, and shortening the life span thereof, for example.
Even when the components are precisely produced, if the stiffness of the rod 42 is low, because of the small clearance between the rod 42 and the slide hole 31, the rod 42 may come into contact with the slide hole 31 at the time of assembly or disassembly, producing sliding friction.
It is therefore an object of the present invention to provide a highly efficient, highly reliable, long-life Stirling engine.
It is another object of the present invention to provide a Stirling engine that offers good workability by permitting easy assembly and disassembly.